After more playing it feels to me that Shar might be the main culprit, if there is a main. But the dead Three are certainly involved in some way. It may be they made some kind of a pact with Shar, because they are not as strong and powerful anymore, and Shar has her own issues and goals. Several characters describe Absolute as female and i dont think the dead three would easily go for such pretending.
The devils are obviously an outside force in this. Its not their game but they do have interests in it.
Apart from that the only unclear thing is whats the deal with presumed Illithid diety. Is it an actual deity or an impostor.
The mural that is the first thing we see in the game makes it definite there is some sort of religious thing going on with Illithids,although it may be just a part of Illithids, not all of them.
There is an elder brain out of whom the chosen one comes forth holding the special staff above kneeling and bowing multitudes.
I have no idea what are the things on the left and right of the figure with the staff, the creatures with two big legs and long tails, bent so their big triangular heads touch the floor. Anyone knows?
On a second look, those might be humans leaning back with their shoulders but their heads are unclear. The tentacles , not tails, would then be behind them.
The crowd is also humanoid on the left and right and only the middle row are Illithids.